1. Field of the Invention
This invention relates to a picture signal processing device for picture signal processing, such as conversion of the number of pixels or conversion of the scanning lines.
2. Description of the Related Art
Recently, digital signal processing has come to be used in keeping up with recent progress in semiconductor technology and increased semiconductor processing speed. Also, recently, a fixed pixel display device typified by a liquid crystal display device or a plasma display device is in widespread use to take the place of a conventional CRT.
In addition, it has recently become a desideratum to be able to display not only the standard television broadcasting system, such as a so-called NTSC (national Television system Committee) signals, PAL (Phase Alternation by Line), but also signals of various formats, such as video graphic array (VGA) signals or SVGA (super VGA) signals or XVGA (Extended VGA) signals.
The numbers of pixels handled by these various formats differ from one format to another. For displaying picture signals of various different formats having different numbers of pixels by an analog display device, such as CRT, it suffices to change the speed of deflection of the electron beam depending on the number of pixels per scanning line.
However, since the number of pixels that can be handled by the above-mentioned fixed pixel display device is set, the conventional analog technique, such as the CRT, cannot be used. Thus, for displaying signals of the above-mentioned different formats on the fixed pixel display device, conversion of the number of pixels or conversion of scanning lines to optional values is mandatory.
The outline of the above-described processing for conversion of the number of pixels is hereinafter explained.
The processing for conversion of the number of pixels is the processing for increasing or decreasing the number of output pixels to a desired number with respect to the number of input pixels during one scanning line period. Supposing that the sampling frequency of the input is equal to that of the output, the increased number of the pixels or the decreased number of the pixels are tantamount to an enlarging processing or to a contracting processing of an input picture (processing of changing the number of pixels in an increasing direction or in a decreasing direction). If attention is directed to the sampling of the input and output pixels instead of to the number of pixels, the above technique is tantamount to creating data at points different from original sampling points or to generating interpolated pixels at these different points from the input pixel data.
The conversion processing for the number of scanning lines for interlaced and non-interlaced pictures is hereinafter explained.
The conversion of the numbers of scanning lines is a processing of changing the number of output lines to a desired value from the number of the input scanning lines during each vertical scanning period (processing of converting the number of lines in an increasing direction or in a decreasing direction). Supposing that the number of lines of the input is equal to that of the output, the increased number of the lines or the decreased number of the lines are tantamount to an enlarging processing or to a contracting processing of an input picture in the vertical direction. Thus, the conversion processing for the number of scanning lines means interpolation of line data.
There are a variety of interpolating methods, which may roughly be classified into the following three methods:
1. Nearest Neighbor Interpolation Method
This method consists in picking up data at the closest position to the pixels following pixel number conversion from pixel data of an input picture, and can be implemented by a logic processing using an extremely simple hardware structure. However, the the picture quality is seriously deteriorated after conversion. Specifically, after contraction, fine lines tend to disappear, or small-sized figures become distorted, whereas, after enlargement, jaggies tend to be produced in the peripheral portions.
2. Bi-linear Interpolation Method
This method, which picks out from pixel data of an input pixel data at two points closest to the position of the pixel following conversion of the number of pixels of an input picture to effect linear interpolation of the two points of data, undergoes picture quality deterioration to a lesser extent than the nearest neighbor method. However, on contraction to less than 2:1, the phenomenon known as pixel drop occurs to deteriorate the picture quality drastically. This technique is tantamount to moderate low-pass filtering so that the entire picture, in particular its edge portion, becomes blurred. The hardware structure is drastically complicated as compared with that for the nearest neighbor method.
3. Filter Switching Interpolation Method
This method, used for picture signal processing for high-quality pictures, converts the number of pixels using a digital filter of a FIR filter, such as a finite response filter, matched to the size conversion ratio. If this method is implemented by a hardware, the hardware structure is drastically complex and enlarged, so that the usual preference is the bi-linear interpolation method.
However, for accommodating the above-mentioned various formats, and under a situation in recent years in which new formats are proposed one after another, the above-mentioned ASIC cannot be designed as a product conforming to the market needs because of its circuit scale or because of poor flexibility such as flexibility in changing bit fineness after designing or addition of the design parameters of the new formats. That is, if desired to implement pixel number conversion by ASIC, the conversion ratio is low in the degree of freedom or remains fixed, or a number of conversion ratios need to be used in a switching manner. Also, with ASIC, bit fineness cannot be changed with ease once the circuit is completed. In addition, it is virtually impossible to cope with various formats including not only the above-mentioned various signal formats, such as VGA, SVGA, XVGA or HDTV, but also various other formats that will make their debut in future.